Retardation of unimolecular reactions in the solid phase

Abstract

The retardation coefficient, K R = k l/k s, for a homogeneous reaction in the bulk of a crystal was calculated within the framework of the free-volume model of unimolecular reactions in the solid phase in the approximation of an isotropic continuum. The key parameters entering into the working formula, more specifically, the additional activation volume for the solid phase and the compressibility coefficient, were estimated using semiempirical methods. The calculated values of K R range from several units to several thousand units, an interval that encompasses all the experimental values of this quantity. Low values of K R are indicative of reactions in the bulk of the crystal lattice. In addition, reactions on defects can occur, being predominant at K R > 100. In all cases, the calculated values of K R give an upper estimate of the degree of retardation, above which no experimental values have been obtained.